1. Field of the Invention
The invention provides a novel dowel machined from the cortex of bone diaphyses and methods of use thereof.
2. Background
It is common for patients presenting with spinal trauma or pathology to require the fusion of two or more vertebra. In the art, a standard solution to this problem is to create a cavity between two adjacent vertebra to accept the insertion of a dowel made from bone or another material. For this purpose, a dowel known as the Cloward Dowel has been in use for many years. That device is a generally circular pin made by drilling an allogeneic or autogenic plug from the cancellous bone of the ilium (i.e., the hip bone). As such, this bone has two cortical surfaces (i.e., it is bicortical) and has an open, latticed or porous structure between the two cortical surfaces. Unfortunately, such dowels have very poor biomechanical properties, principally being susceptible to compression. Accordingly, such dowels present the major danger of collapsing prior to fusion of the adjacent vertebra between which such a dowel is inserted.
A dowel of greater biomechanical properties has been produced from allogeneic femoral or tibial condyles (i.e., the rounded prominence at the end of the femur or tibia where such bones articulate with other bones). The result of drilling a plug from such a condyle is a unicortical dowel. Such unicortical dowels are available from most tissue banks, including the University of Florida Tissue Bank, Inc. (see, for example, our Allograft Catalog, product numbers 280012, 280014, and 280016; this catalog and these products are available on request by calling 904-462-3097, or by calling 1-800-OAGRAFT, or by writing to the University of Florida Tissue Bank, Inc. 1 Progress Boulevard, P.O. Box 31, S. Wing, Alachua, Fla. 32615). While such unicortical dowels represent a major advance over the bicortical dowels of Cloward, desribed above, from a biomechanical point of view, the biomechanical properties of the diaphysial cortical dowel of the instant invention is expected to represent a substantial improvement over the unicortical dowels, due to the greater density of source bone, as will be evident from a reading of the full disclosure which follows.
In addition to the known Cloward and unicortical dowels, a number of U.S. Patents have been found dealing with the general area of dowels for achieving vertebral fusions. Thus, for example, U.S. Pat. No. 5,015,247 discloses a threaded spinal implant which, when placed between two adjacent vertebrae, directly participates and is incorporated in the ensuing fusion. The implant is made of a hollow metal casing which is filled with osteogenic material. A plurality of perforations are provided in the casing so that bone can grow into and out of the implant. Metal threads and tabs are provided to insert and prevent backing out of the implant, respectively. However, the implant is made out of metal and thus is a foreign object which is inserted into the spine and is thus never fully incorporated into the fusion. Furthermore, as the implant is preferably made of titanium, production of the implant requires the use of specialized metal molding and machining, and production of the implant material itself, which is expensive.
In U.S. Pat. No. 4,627,853, a method of producing a prosthesis for replacement of articular cartilage and the prostheses so produced is disclosed. The prostheses of the '853 patent, principally designed for articulating cartilage replacement, are machined from allogenic or xenogeneic bones segments and then demineralized to produce a bone fragment with a spongy texture similar to natural cartilage. The prostheses are also tanned to render the material non-antigenic. While the methods of the '853 patent may be used to alter the properties of the diaphysial cortical dowel of the instant invention, and the disclosure of the '853 patent is herein incorporated by reference for that purpose, the '853 patent does not teach or suggest the novel device and method of the instant invention.
In U.S. Pat. No. 5,053,049, a flexible prosthesis and a method for making such prostheses are disclosed. The process includes machining a bone, demineralizing the bone to impart a desired degree of flexibility, and tanning to render the material non-antigenic. This patent is generally similar in disclosure to the disclosure found in the '853 patent discussed above, except that the particular applicability of the disclosed process to the production of an outer ear prosthesis is emphasized.
In U.S. Pat. No. 5,306,303, a bone induction method is disclosed which consists of implanting a bone morphogenetic, protein-free ceramic in the soft tissue or bone of an animal. The ceramic disclosed as preferable is calcium phosphate and the use of such material for achieving spinal intervertebral joint fusions (disk arthroplasty) is suggested. The material and product of the '303 patent, aside from its possible use for a purpose similar to that for which the instant product is designed, bears little or no resemblance to the instant invention.
In U.S. Pat. No. 5,171,279, a method for subcutaneous suprafascial pedicular internal fixation of vertebrae of the spine is disclosed to facilitate graft fusion. The method included excision of the nucleous of an affected disc, preparation of a bone graft, instrumentation of the vertebrae for fixation, and introduction of a bone graft into the resected nuclear space. Metallic fixation hardware is disclosed as the principal aspect of the claimed invention. Accordingly, aside from dealing with the same general problem, the invention disclosed and claimed in the '279 patent bears little resemblance to the diaphysial cortical dowel and method of the instant invention.
Accordingly, having reviewed many solutions attempted in the field prior to the instant disclosure, it is concluded that there remains the need for a vertebral fusion graft which has superior biomechanical and vertebral fusion promoting properties. The instant invention provides such a graft as well as a method for making and using the graft.